Aluminum belongs to a mainstay industry of the non-ferrous metallurgical industry in China; by 2013, China's primary aluminum production capacity reached 18 million tons, with aluminum oxide production capacity close to 40 million tons, ranging the top in the world. At present, more than 80% of aluminum oxide in China is produced by using the Bayer method: the silicon-containing phase in bauxite can be converted to hydrated sodium aluminosilicate (Na2O.Al2O3.1.7SiO2.nH2O) in aluminum oxide production with the Bayer method, namely, 1 kg of silicon oxide in minerals enable 1 kg of aluminum oxide to be removed from red mud and cause an loss of 0.608 kg of alkali. Containing alkali and aluminum, red mud has two problems: first, high content of the alkali makes the red mud not usable in cement and other bulk industries; second, during treatment of low-grade bauxites, the losses of aluminum oxide are too large, so that the overall yield is lower.
In order to achieve efficient utilization of the red mud and extraction of valuable elements, Chinese aluminum industry participants have been doing a lot of research and development work. The conventional utilization technologies for the red mud are generally divided into two types: one type is to use the red mud overall as a general industrial raw material, for example, in patent application numbered 200910303512 and titled “High-Volume Flyash Cement and Preparation Method thereof”, invented by Zhang Kaiyuan et al., flyash, red mud, lime, cement clinker, gypsum and admixtures are used as raw materials, the red mud is dried, and then the dried red mud and the cement clinker are mixed and ground together; the lime and the gypsum are crushed; the admixtures are prepared into a solution; the flyash, the red mud, the lime, the cement clinker, the gypsum and the admixtures are uniformly mixed and finely ground to obtain the high-content flyash cement; and another example, in patent application numbered 200710105971 and titled “Process and Method for Completely Converting Aluminum Industrial Process Waste Residues into Ecological Building Materials” invented by Wang Wenju et al., the inherent substance properties of six kinds of waste residues generated in the production process of the aluminum industry—solid waste red mud (sintering method and Bayer method), boiler slags, beneficiation tailings, cinder, gas slag and sludge—are utilized to convert waste residues into novel road construction materials and building wall materials through processes of drying, crushing, reasonable proportioning, processing shaping (rolling & extrusion), solidifying or sintering.
The other type is to respectively extract valuable metal elements therein, in which the most typical manner is treating the Bayer red mud by a sintering method, or by an acid leaching method. For example, in the patent application numbered 201010561605 and titled “Process Method and Equipment for Separating Iron and Aluminum Silicon Residues and Removing Alkali Metal from Red Mud” invented by Dong Yafei et al., red mud, coal power, lime and adhesives are uniformly mixed in proportion so as to obtain a mixture, and the mixture is compressed to obtain granules; after drying, rotary hearth furnace smelting reduction, grinding and magnetic separation, iron and aluminum silicon residues are separated out; the separated iron is used for electric steel making or steel casting and the like, while the aluminum silicon residues are used for the production of high-grade and good-quality cement or as raw materials of refractory materials.
In the conventional technologies of using red mud, direct usages generally have problems of low product prices, low profits, and the like. Extracting valuable elements separately usually has problems of high energy consumption, high equipment requirements and the like during treatment. Thus, currently industrialized red mud is treated mostly by direct stockpiling. Although some aluminum oxide production enterprises dealkalize red mud and use the dealkalized red mud for cement and other industrial applications, there are two problems in direct dealkalization: first, when dealkalization is performed on the red mud with lime, only sodium oxide can be recovered; although the alkali content in red mud can be reduced to the level as required by the cement industry, since the products are monotonous, the overall benefit is low; second, low-concentration alkaline solutions produced after dealkalization of red mud in low-concentration alkaline solutions or clear water cannot be directly used and is required to be concentrated by evaporation, therefore the energy consumption is high.